The present invention relates to a method of designing a corrective lens, particularly to an aspherical spectacle lens provided with a rotationally asymmetrical optical property, such as, a cylindrical power or a prismatic power.
Design methods for designing an aspherical spectacle lens provided with a cylindrical power have been proposed. Examples of such a design method are disclosed in International publication No. WO93/07525 and Japanese Patent publication NO. 3236263. As shown in the above mentioned publications, the conventional design method requires the use of very complicated equations regarding an aspherical surface to be designed and the use of a number of parameters. Therefore, it is very difficult for a designer to estimate an effect on optical performance resulting from changes of values of the parameters. For this reason, generally a computer-based automated design method by use of an optimization program is used for design of an aspherical spectacle lens.
However, such an optimization program requires matrix calculation having at least the size defined by (the number of parameters)×(the number of aberrations to be evaluated). Therefore, the processing time and the program size of the optimization program increase with the increase of the number of parameters. The term “aberrations to be evaluated” means aberrations to be corrected in the process of the automated design program in which target values of the aberrations are established and the evaluated values approach the target values. In addition, the larger the number of parameters becomes, the larger a possibility that the optimization program tends to be brought to a state of a local minimum becomes. There is a possibility that the optimization program can not reach true optimum values.
Furthermore, the use of the optimization program requires a designer to establish target values of the aberrations in accordance with specifications of a lens to be designed. Therefore, whether or not the target values reach true optimum values depends on the judgment of the designer. In particular, if the lens to be designed is a rotationally asymmetrical lens, a designer is required to be considerably careful about the judgment.
Furthermore, there is a possibility that a result of automated design varies among designers who address the automated design for lenses having the same optical performance.
Furthermore, a design method suitable for designing an aspherical spectacle lens having a prismatic power has not been proposed. Conventionally, such a lens is designed in such a manner that a first surface and a second surface of an aspherical lens, which was designed not to have a cylindrical power, are inclined with respect to each other so as to provide a prismatic power for the aspherical spectacle lens. If such a design manner is used, the occurrence of aberrations is unavoidable.